Plastic-based sheet materials, e.g., sheet materials formed of plastic, plastic fibers, or blends of plastic and other materials, and laminates of such materials with other types of sheet materials, are used for packaging, currency, identification cards, passports, credit cards, and many other applications. When such materials are used in currency, the currency is often referred to as “polymer banknotes.”
In some applications, for example when these sheet materials are used as currency, in credit cards, and in other financial applications, it is often desirable to be able to “mark” or “tag” the sheet material with a special marking that is not visible to the naked eye, and/or cannot easily be produced by counterfeiters. Marking can be used, for example, to prevent or detect counterfeiting. Marking can also be used on currency to allow the currency to be traced and/or identified, e.g., if it is stolen or used in a criminal transaction.